This concise guide explains how a grinding circuit for high-purity gold recovery works; it covers machine principles, key parameters, operational data, maintenance, two verified project cases, and practical selection advice to minimize gold loss and protect yield.
A gold grinding machine reduces ore to a controlled fineness so that downstream leaching or flotation recovers free gold. The goal is to liberate gold particles while minimizing over-grinding that causes fine loss. In practice, operators set targets for feed, grind size, and throughput to match separation steps. Moreover, a stable circuit reduces variability, and therefore reduces gold loss.
Grinding equipment includes a primary crusher, a SAG or ball mill, classifiers (hydrocyclones or screens), and drive systems. The mill rotates; feed, grinding media and ore interact; particle size reduces by impact and abrasion. The classification loop returns coarse material. Critical control variables include mill speed, mill loading, grinding media ratio, and classifier cut-size. These variables determine energy efficiency and liberation.
Operators must consider: 1) Feed and product size (P80), 2) Specific energy (kWh/t), 3) Mill diameter and length ratio, 4) Critical speed (rpm), 5) CSS/OSS (closed/open side settings for crushers and screen gaps), 6) Crushing ratio, 7) Motor power and gearbox selection, 8) Throughput (t/h). For example, a ball mill’s P80 output 80% passing size must match leach requirements; industry practice often targets P80 75–150 μm for free-milling gold ores, while refractory ores may require finer milling. ; however feed characteristics change this range.
Choose a motor that provides torque reserve. Motor power must match design throughput at the desired grinding media load. A typical rule: select a motor with 10–20% power margin above steady-state draw. Shaft design, coupling, and pinion-gear alignment control vibration. For example, a 3.5 m × 6.0 m ball mill often pairs with a 500–900 kW motor depending on ore hardness and desired t/h. Proper VFD or soft-start reduces mechanical stress, and ensures controlled ramp-up, thus reducing spalling and maintenance cost.
Track these KPIs: specific energy (kWh/t), circulating load (%), mill availability (% uptime), maintenance interval (hours), and unplanned downtime. A well tuned circuit shows specific energy 12–18 kWh/t for free-milling gold ore; pumps and cyclones account for an additional 0.5–1.5 kWh/t. Average fault rate should not exceed 0.5 incidents per 1000 operating hours for key drives. Maintenance cycles for liners run 3–6 months depending on abrasion; bearing re-grease interval often every 500–1000 hours; gearbox oil change typically 2000 hours. These practical numbers reflect field experience and industry reports, and they guide budgeting and scheduling. ,
Below is a representative equipment list for a 100 t/h free-milling gold plant. The list shows machines commonly used and matched specifications to reduce gold loss and ensure process balance.
| Item | Model/Spec | Key parameter |
|---|---|---|
| Primary crusher | Gyratory / SBM small jaw | Feed 500 mm; reduction ratio 6:1 |
| SAG/Ball mill | Ball mill 3.2×7.0 m | Motor 800 kW; P80 120 μm |
| Classifier | Hydrocyclone cluster | Cut size 100–150 μm |
Use Bond’s law for energy estimation. For rough planning: E = 10·Wi·(1/√P80 – 1/√F80). Here E is kWh/t; Wi is Bond work index (kWh/t), F80 and P80 are feed and product sizes in microns. For a typical ore with Wi = 12 kWh/t, feed 1200 μm and target P80 120 μm, E approximates 12 kWh/t. Thus choose motor and mill dimensions to supply that energy continuously. Operators should validate with a closed-circuit test before final design. ,
First, avoid over-grinding. Second, control cyclone overflow size and density to match the downstream leach. Third, maintain a stable circulating load. Fourth, monitor slurry density and pH as they affect flotation and cyanidation. Additionally, grade control and feed blending reduce variability. Real-time particle size monitoring helps; it reduces tail loss by enabling quick correction of mill speed or media charge. moreover, proper sampling protocols improve mass balance accuracy and reveal hidden losses early.
Common faults: gearbox wear, liner damage, bearing overheating, seal failure. Preventive maintenance schedules and predictive vibration monitoring reduce unplanned stops. Expected lifecycle: liners 1–3 years depending on abrasion; motors and gearboxes 10+ years if maintained. Energy accounts for 60–70% of operating cost in grinding circuits; therefore energy-efficient designs yield best ROI. Keep spare parts kits for pinions, seals and bearings on-site to reduce downtime; plan major overhauls yearly or bi-yearly depending on hours.
Project background: open-pit ore; soft, oxide gold ore; feed 0–25 mm; plant capacity 120 t/h; climate: humid subtropical; location: eastern province. Design choice: primary jaw crusher, 1 SAG mill + 1 ball mill in series; hydrocyclone cluster; gravity concentrator ahead of leach. Key problem: fine gold slip through flotation. Solution: installed high-efficiency gravity concentrator (centrifuge) before leach; refined cyclone cut; set P80 to 100 μm. Results: gold recovery improved from 89.2% to 94.5% in six months. Client feedback (SBM equipment): praised quick commissioning and stable throughput; noted easier liner changes and clear documentation; operators reported lower gold in tailings samples. ,
Project background: hard, sulfide-rich ore; particle liberation required fine grind; feed 0–50 mm; cold climate. Design choice: upgraded ball mill, increased motor power to 800 kW; added high-pressure grinding rolls (HPGR) at feed; enhanced cyclone control. Problem solved: excessive over-grind and refractory gold locked in sulfides. Outcome: overall recovery jumped from 65% to 78% after retrofit and pre-oxidation; energy per tonne dropped slightly due to HPGR pre-breakage. SBM field report: installation time matched schedule; maintenance training reduced early faults; plant achieved steady specific energy within design band.
Step 1: Define ore type (free-milling vs refractory). Step 2: Determine feed size and target P80. Step 3: Calculate energy using Bond method. Step 4: Match mill size and motor power; add 10–20% margin. Step 5: Select classification technology (cyclone or screen). Step 6: Include gravity recovery when coarse free gold present. Step 7: Plan spare parts and maintenance. If energy requirement >20 kWh/t; consider staged grinding with HPGR or SAG to reduce cost.
We advise start-up with empty mill media, slow ramp to design speed, and gradual feed increase. Use alignment jigs during coupling install; verify grease intervals; implement vibration baseline checks within first 100 hours. Provide operator training and spare part kits. A well-documented handover reduces first-year faults. moreover, schedule liner checks at 500–1000 hours and gearbox oil analysis quarterly. ,
Q1: What P80 should I target to avoid gold loss? A: For free-milling ores: P80 75–150 μm. For refractory sulfide ores: P80 often <75 μm. Test with locked-cycle milling to confirm.
Q2: How to detect hidden gold loss? A: Use mass balance, check tail assays, run diagnostic gravity concentration, and particle size analysis. Regular tailings checks every shift help catch loss early.
Q3: Is more mill power always better? A: No. Excess power causes over-grinding, creating slimes that lock gold. Match power to required specific energy; add margin, not surplus, and use correct classification.
Choose machines sized to calculated energy demand. Use gravity ahead of chemical recovery for coarse gold. Control P80 and circulating load tightly to avoid loss; conduct test-work before final design. Prioritize alignments, motor sizing, and classifier control. Finally, field-verified case studies show that correct pre-treatment and classifier tuning deliver measurable recovery improvements, and reduce operational risk. SBM customers report smoother commissioning and reduced tail losses after following these steps.
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